The purpose of this study is to quantitatively evaluate the post - and pre synaptic molecular aspects of synaptogenesis and to relate these phenomena to the differentiation of fast and slow twitch mammalian muscle. This will be accomplished by analyzing end-plate currents and the statistical parameters of transmitter release and comparing these to the appearance of fast-twitch and slow-twitch phenotypes characteristic of the extensor digitorium longus and the soleus muscles, respectively. The specific objectives of this project are: 1) to determine the role of axoplasmic flow and sensory input, via muscle spindles, upon the sensitivity of muscle fibers to hyperneurotization; 2) to compare and contrast the sequence of events underlying synaptogenesis during development and reinnervation with that seen during synapse degeneration early after crush of the muscle nerves and during elimination of foreign innervation subsequent to reinnervation by the original nerve; 3) to relate synaptic maturation to the appearance of fundamental electrophysiological, pharmacological and mechanical properties characteristic of fast and slow muscles.